A phylogenetically-controlled test of habitat effects on plant anti-herbivore defenses: Constitutive and induced resistance

The growth rate (GR) hypothesis relates the evolution of plant defense to resource availability and predicts that plants that have evolved in abiotically stressful environments grow inherently more slowly and are more constitutively resistant to herbivory than plants from more productive habitats.  Stress-adapted plants are also predicted to have reduced inducibility, but this prediction has not been previously tested.  To evaluate this hypothesis, I compared the growth of nine species of herbaceous plants from Missouri glade habitats to congeners from more productive non-glade habitats.  I also conducted bioassays using larvae of the generalist herbivore, Spodoptera exigua to estimate constitutive and inducible resistance in these congeners.  Glade congeners tended to grow more slowly and have higher constitutive resistance and lower inducibility than non-glade species.  However, none of these comparisons were statistically significant due to the conflicting response of one congeneric pair (Salvia azurea and S. lyrata).  Analyses without this genus were consistent with the GR hypothesis, as were analyses that categorized congeners by relative growth rate.  These results highlight the complexity in searching for factors that determine plant growth rates and resistance traits across multiple genera and support the hypothesis that both constitutive and induced resistance may be influenced by selection on traits that alter plant growth rates.  Future studies should attempt to determine whether variation in inducibility is better explained by habitat or relative plant growth rates.